The present invention relates to a sampling device for obtaining a sample of an analyte, and to a method for obtaining a sample of an analyte.
In process engineering and medicine, in particular when observing a bioreactor and during the long-term determination of the content of preselected substances in the human body, it is often necessary to rapidly determine as continuously as possible the presence and, if necessary, the concentration of a preselected analyte in a medium. In this respect, it is often impossible for hygiene and/or medical reasons to directly take material of the medium to be examined, for example in a biopsy, in a repeated or continuous manner. For this reason, a sample of the analyte is usually obtained from the medium by a dialysis process. In such a process, a probe provided with a dialysis or gas diffusion membrane as the “measuring window” is introduced into the medium to be examined and is optionally implanted for a relatively long time. The probe is flushed continuously or in pulses with a transport medium. The analyte passes through the membrane into the transport medium of an analyte feed chamber positioned after the membrane in the analyte flow direction and is transported through the probe through a probe outlet out of the region of the medium to be examined, in particular a bioreactor or a human or animal body. The transport medium which may be loaded with analyte can then be analyzed by one or more sensors. Examples of such probes and associated sampling methods are described in DE 44 26 694, U.S. Pat. No. 3,640,269, U.S. Pat. No. 4,694,832, U.S. Pat. No. 6,632,315 B2, U.S. Pat. No. 6,811,542 B2, WO 99/45982 A2 and WO 01/06928 A1.
A problem of previous probes and of corresponding sampling methods was the restricted transportation speed of the analyte in the probes and accordingly the lengthy dead time between the first passage of the analyte through the membrane and the first detection of the analyte at the probe outlet. For example, the cardiac catheter described in WO 99/45982 A2 requires a dead time of 15 to 20 minutes before a measurable signal is present; more on this below.
To solve this problem, it was attempted on the one hand to increase the membrane surface area. For this purpose, microdialysis probes, for example were used which comprise hollow fibers with an individual fiber diameter of approximately 500 μm. A disadvantage here is that when the probe is withdrawn, particularly from tissue, the membrane can become detached and can remain in the body. To achieve a high transportation speed, microdialysis probes are often charged with a high internal pressure of the transport medium. The high internal pressure also stresses the connections between the hollow fibers and the rest of the probe, so that tears often appear in the hollow fibers or the hollow fibers tear out of the probe.
In addition, the pressure fluctuations which may occur during conventional operation of a bioreactor or during implantation of a probe in a human or animal patient body give rise to strong fluctuations in the dialysis rate. This effect is further intensified in that the used membrane materials swell considerably in aqueous surroundings and consequently become very flexible. Thus, when hollow fibers are used, after implantation or immersion into a measuring solution, “tubular bags” which are very pulsatable are produced as measurement windows. The dialysis rate then fluctuates considerably because the membrane cell volume can constantly change internally with a varying counter pressure. Even the smallest movements or fluctuations in pressure can be clearly detected. A remedy can only be provided by operation under elevated internal pressure, which again increases the risk of rupture. Accordingly, such microdialysis probes are generally unsuitable for a pulse-wise loading of the probe with the transport medium.
The cylindrical dialysis chambers which result when hollow fibers are used suffer from disadvantages from a fluid-dynamics point of view as well. Produced in the analyte feed chamber are regions, corners and reversals in direction which are flown through in different ways depending on construction and in which air bubbles can be deposited. Since microdialysis probes usually comprise analyte feed chambers of a small internal diameter, only slow flow rates can build up inside, so that it is very difficult to flush out the air bubbles. The air bubbles alter the volume of the analyte feed chamber filled with transport medium and/or the effective membrane surface area and thus give rise to considerable measurement errors.
And finally, the maximum volume flow of the transport medium in a microdialysis probe is restricted due to the desirable high substance transportation rate of the analyte into the analyte feed chamber. Conventional microdialysis probes can only be operated at transport medium volume flows of 0.3-0.5 μl/min. Thus, for the transportation of the transport medium out of the analyte feed chamber to the probe outlet, with an assumed transportation path of 30 cm and a conventional probe internal diameter of 75 μm, a time (dead time) of 160 s is required and with a probe internal diameter of 150 μm, a time of even 630 s is required. By reducing the probe internal diameter, the pressure of the transport medium at the same transportation speed increases by a factor of 4, so that a minimum internal diameter of the probe must be observed to prevent the membrane from tearing or bursting.
Sampling probes are also known in which the transport medium is not exchanged continuously but in pulses. In this case, the analyte diffuses out of the medium to be investigated also through a membrane (dialysis or gas diffusion membrane) into an analyte feed chamber positioned after the membrane in the diffusion direction, into the transport medium. Depending on the length of time between two consecutive transport medium exchange pulses, the analyte is concentrated to different degrees with the same starting analyte concentration of the medium to be examined in the analyte feed chamber. For example, U.S. Pat. No. 6,852,500 describes a microdialysis probe and a glucose flow sensor, a transport medium flowing in pulses through the microdialysis probe. Bearing in mind the internal volume of the microdialysis probe, there results a period of 9 minutes until the entire volume of the analyte feed chamber has passed through the probe outlet and has reached the glucose flow sensor. The measured results determined thus can therefore only be averages and merely reflect the actual course of the glucose concentration in a time-delayed and damped-down manner.
Therefore, the object of the present invention was to provide a sampling device by which the disadvantages described above of conventional sampling devices can be avoided or reduced. The sampling device should in particular allow a short dead time between the entering of an analyte into an analyte feed chamber and passage through the sampling device outlet. A transport medium should preferably be able to flow through a course 30 cm in length between the analyte feed chamber and the sampling device outlet within 2 minutes, without pressures arising which can threaten the reliable operation of the sampling device and in particular initiate tearing of the sampling device. Furthermore, a corresponding sampling method is provided.
The object is achieved by a sampling device according to claim 1. Advantageous developments of the invention are described in the subclaims. A sampling method is described in claim 17.